This invention relates to cylinder locks which are responsive in different ways to different keys. Such a lock is useful in applications where certain persons may be authorized to lock, but not unlock, a door. Such a situation arises in a commercial or industrial setting where an entrance door would be unlocked during working hours, but locked during non-working hours. The owner of the premises may desire that an employee be able to lock a door at the end of the day, but not have the ability to unlock it thereafter, the ability to do both being reserved to the owner. It is thus desirable to have a lock responsive to two keys, both of which can perform the locking operation, while only one of which can perform the unlocking operation.
Prior art devices directed to this problem will be seen in the following U.S. Pat. Nos.:
3,771,340 PA1 3,875,773 PA1 3,973,421
In the prior art devices wafers or discs are used as tumblers. Neither of these arrangements is thought to be as secure as the pin-tumbler lock employed in the device embodying the present invention. An additional and important difference between the present invention and those disclosed in the above listed prior art patents is that the present invention may be utilized in standard pin-tumbler locks, including those already installed, by merely substituting the core plug described hereinafter for the typical core plug and using a subordinate key. In this manner, ease and economy of manufacture and installation are assured.